400 BROOKLYN BOTANIC GARDEN MEMOIRS 
The notion of bridging hosts, of course, implies that the fungus 
undergoes a physiological change in consequence of its new habitat 
and thus becomes able to attack other hosts. The change is certainly 
closely associated with variation in virulence so well known in the 
case of pathogenic bacteria. It is quite likely that fungous parasites 
vary in virulence or can be made to do so by suitable experimental 
methods. At present, however, we have no good evidence that this 
has been done in any particular case. The results of Evans (44), 
referred to above, point in this direction. In this case, however, the 
data are not at all extensive. The facts might be explained by an 
increase in susceptibility of both resistant and susceptible parent in 
response to a change in the environment; or external factors may 
have favored a more successful invasion on the part of the rust parasite. 
Passing reference may be made to the work of Salmon (129, 130), 
Ward (173), Stakman (143) and Spinks (141) which clearly indicates 
that a plant may be rendered more or less susceptible to fungous 
invasion by means of certain agencies. The work of these investi- 
gators shows that mineral starvation, excess of nutrients, mechanical 
injuries, anaesthetics, etc., modify the relations of a plant to fungous 
invasion. 
It has been pointed out by Eriksson (35), Ward (172, 174), Vavilov 
(164, 165) and others that a specialized race tends to occur on more or 
less closely related hosts. There are, however, great differences among 
the specialized races in this respect. As pointed out above, the host 
range of these races may be narrow or wide. Within a single species 
of parasite we may have a race occurring on many hosts belonging to 
different genera and another race restricted to a single genus or even 
certain species of a genus. Puccinia graminis, as well as other fungi, 
includes races of such wide differences in host range. 
Attempts have been made to utilize the infective capacity of a 
parasite to determine the genetic relationship of hosts. Eriksson (35) 
applied this test in determining the possible relation of a rye-wheat 
hybrid to the two parents. Ward (171, 172) reports a fairly close 
correspondence between the hosts of the more or less well-defined 
races of Puccinia dispersa bromi and the grouping of the bromes on 
other grounds. Vavilov (154, 155) has used Puccinia dispersa tritict 
and Erysiphe gramints tritici as a test in determining the relationship 
of types and varieties of Triticum. He also used Puccinia graminis 
avenae and P. coronata avenae as a similar test in connection with species 
and varieties of Avena. It is interesting to note that the parasites on 
wheat gave practically the same results and these are both quite 
narrowly specialized races. The rusts on oats, however, did not give 
corresponding results, Puccinia graminis avenae infecting a wider 
